Refactor notification/email inheritance structure

[williamjallen]

While initially developed with good intentions, the email notification system has become a major pain point for developers due to the convoluted hierarchy of classes and interfaces. A better solution for handling notifications is Laravel's built-in notification system. This PR deletes many unnecessary interfaces, adds type annotations to notification code where possible, and otherwise cuts down on unnecessary technical debt.

[fspindle]

To be able to use smtp I had to modify config/mail.php setting verify_peer to false.

        'smtp' => [
            'transport' => 'smtp',
            'host' => env('MAIL_HOST', 'smtp.mailgun.org'),
            'port' => env('MAIL_PORT', 587),
            'encryption' => env('MAIL_ENCRYPTION', 'tls'),
            'username' => env('MAIL_USERNAME'),
            'password' => env('MAIL_PASSWORD'),
            'timeout' => null,
            'local_domain' => env('MAIL_EHLO_DOMAIN'),
	    'verify_peer' => false,
	],

[williamjallen]

@fspindle Just to clarify, that behavior is not different between my branch and master, correct? This is a maintenance PR which should not have any effect on the current behavior.

[fspindle]

Yes, you are right. The behavior is the same as in the master branch. I just wanted to share that adding 'verify_peer' => false makes email notification working like adding the same line to the master branch (See #2618)

[josephsnyder]

I don't send any emails from my test installation but the tests pass and the code seems in good shape. LGTM